The present invention relates to milled carbon fibers. More particularly, the present invention is concerned with milled carbon fibers which have a large surface area available for contact with metals, etc., so that it is suitable for improving the rigidity and high-temperature heat resistance of metals, alloys and the like, thereby ensuring advantageous utilization thereof in, for example, carbon-fiber-reinforced composite materials. Also, the present invention is concerned with a process for producing the milled carbon fiber.
The carbon fiber is lightweight and has high strength and rigidity, so that in recent years it is utilized in a wide spectrum of fields from the aerospace and aircraft industry to the general industries.
For example, carbon-fiber-reinforced plastics are actually widely utilized as structural materials having high specific strength and specific modulus of elasticity. Further, carbon-fiber-reinforced metals (CFRM), such as carbon-fiber-reinforced aluminum alloys and carbon-fiber-reinforced magnesium alloys (hereinafter referred to as xe2x80x9cCFRAl(Mg)xe2x80x9d), have been developed as materials having excellent high-temperature dimensional stability and thermal deformation resistance, and their use is anticipated as a material for use in structural members for aerospace and aircraft and engine members for vehicles.
However, the production of CFRAl(Mg) has encountered, for example, a problem such that not only is the wettability of the carbon fiber with molten Al (or Mg) poor but also, once the wetting is effected, the carbon fiber reacts with Al to thereby form Al4C3 with the result that the strength of the material is lowered.
The amount of formed Al4C3 is connected with the type of the carbon fiber. That is, the carbon fiber produced by heat treating at a temperature of about 2000xc2x0 C., known as xe2x80x9cgraphitized carbon fiberxe2x80x9d, has a high carbon crystallization degree and a strong carbon-to-carbon bond to render itself stable, as compared with the carbon fiber produced by heat treating at a temperature of about 1500xc2x0 C., known as xe2x80x9ccarbonized carbon fiberxe2x80x9d, so that the reactivity with molten Al alloy or the like is poor, thereby minimizing the formation of carbides, such as aluminum carbide.
Therefore, the mechanical properties of the CFRAl(Mg) are superior when the graphitized carbon fiber is used as reinforcement.
The graphite crystals of the graphitized carbon fiber are generally highly anisotropic from the dynamical, electrical and scientific viewpoints, because the carbons interact each other between the graphite layer planes with only weak intermolecular force while the sp2 carbons are strongly bonded within each of the graphite layer planes (c-planes).
In the so-called monoaxially oriented structure in which the c-planes are arranged parallel to the fiber axis, there may be some mutually different microstructures or high-order structures, depending on the type of the carbon fiber precursor (polyacrylonitrile (PAn), rayon, pitch, etc.).
Of the above precursors, when mesophase pitch with greater graphitizability is used as a starting material, the graphitization is more readily promoted even at the same heat treating temperature to thereby produce carbon fibers having higher modulus of elasticity. Therefore, the use of carbon fibers of high elastic modulus derived from mesophase pitch is especially promising in the formation of a composite with an aluminum alloy and the like.
On the other hand, from the viewpoint of moldability, the use of milled carbon fibers is advantageous in respect of the degree of freedom of molding and molding/working costs, although the molding with the use of lengthy carbon fibers is suitable for producing a fiber-reinforced metal composite having excellent mechanical properties.
The use of the milled carbon fibers in the fiber-reinforced metal composite leads to the increase of the surface area brought into contact with metals. The opportunity of reaction with the metals becomes high as much as the above increase, so that greater attention must be paid to the formation of carbides.
Coating with silicon carbide or precoating with a matrix metal, such as aluminum, at low temperatures has been tried for the purpose of improving the wettability with metals and suppressing the above reaction.
However, these conventional trials have had a drawback in that the efficacy is low for the cost increase involved.
The inventors have made extensive and intensive studies with a view toward resolving the above problems. As a result, they have found that the configuration of the milled carbon fiber, especially the morphology of the surface thereof, has an important relationship with the formation of carbides with metals, and that the reaction of the milled carbon fiber with metals can be suppressed by improving the above configuration. The present invention has been completed on the basis of the above findings.
The present invention has been made with a view toward obviating the above drawbacks of the prior art. Thus, the object of the present invention is to provide milled carbon fibers which have desirably grown graphite layer planes and accordingly a low reactivity with metals, so that it can provide a lightweight and rigid fiber-reinforced metal having excellent heat resistance at high temperatures, and also to provide a process for producing the desired milled carbon fibers.
The milled carbon fibers of the present invention are one produced from mesophase pitch, which have a fiber cut surface and a fiber axis intersecting with each other at cross angles, the smaller one thereof being at least 65xc2x0 on the average.
The milled carbon fibers of the present invention preferably have a specific surface area as measured by the BET method of 0.2 to 10 m2/g.
The process for producing milled carbon fibers according to the present invention comprises the steps of:
melt spinning mesophase pitch to obtain pitch fibers;
infusibilizing the obtained pitch fibers;
milling the infusible pitch fibers as obtained or after a primary heat treatment at 250 to 1500xc2x0 C. in an nert gas; and
subjecting the obtained milled fibers to a high-temperature heat treatment at 1500xc2x0 C. or higher in an inert gas.